The present invention relates to a vehicle behavior control apparatus for controlling vehicle behaviors, and more particularly, to a vehicle behavior control apparatus for automotive vehicles, which enables to accurately detect the vehicle state, based on a steer angle as an input signal from a driver to the vehicle and a slip angle difference between the front and rear wheels as an output signal of the vehicle.
Various apparatus are known for controlling vehicle behaviors. Such apparatus include an electric power steering apparatus, a four wheel steering apparatus, a left-and-right driving force distributing apparatus, and a left-and-right braking force distributing apparatus. Of these apparatus, the electric power steering apparatus informs the driver of road surface information (road reaction force) via a steering wheel, by changing an assist torque, and urges a steering operation of the driver in order to stabilize the vehicle behavior. Besides, in the other apparatus, the vehicle behavior is stabilized by the action from the vehicle. For example, the rear wheels of the vehicle are turned to reduce the deviation of the actual yaw rate with regard to the target yaw rate, in the case of the four wheel steering apparatus, and the distribution of the driving force is changed at the left and right wheels, in the case of the driving force distributing apparatus, and further the distribution of the braking force is changed at the left and right wheels, in the case of the braking force distributing apparatus.
The conventional electric power steering apparatus mainly comprises a steering torque sensor, a control unit, a motor driver, an electric motor and the like. The steering torque sensor detects a manual steering torque to be generated by the driver""s steering operation. The control unit transmits a target torque signal for driving the electric motor in accordance with the steering torque, and provides a motor control signal for controlling the drive of the electric motor on the basis of the target torque signal. In order to swiftly convert a motor current flowing into the electric motor into a current corresponding to the target torque signal, the control unit feeds a signal that is corresponding to the motor current actually flowing into the electric motor back to the target torque signal (negative feedback), so as to control the drive of the electric motor. The motor driver is, for example, composed of a bridge circuit including field effect transistors, and in the bridge circuit, it drives the electric motor with PWM (Pulse Width Modulation) on the basis of the motor control signal. The electric motor then rotates and applies an assist torque to the steering system. The electric power steering apparatus corrects the target torque signal by reducing the target torque signal, in proportion to an increasing vehicle velocity due to a velocity signal detected at a velocity sensor. In other words, in a low-velocity driving, the electric power steering apparatus applies a sufficiently large assist torque to the steering system so as to ease the driver""s steering operation, while in a high-velocity driving, the electric power steering apparatus applies a small assist torque to the steering system, thereby stabilizing vehicle behaviors.
The applicant also discloses in Japanese Patent Laid-open Publication No. HEI-11-152057 an electric power steering apparatus, which corrects the target torque signal on the basis of the angle difference between the front and rear wheel slip angles of the vehicle. In order to collect vehicle information, this electric power steering apparatus comprises a velocity sensor, a turn angle sensor, a yaw rate sensor, a steering torque sensor, and the like. The electric power steering apparatus further comprises a slip angle difference predicting section and a correcting section in the control unit. The slip angle difference predicting section calculates the slip angle difference between the front and rear wheels, based on a velocity signal from the velocity sensor, a yaw rate signal from the yaw rate sensor, and dimensional parameters of the vehicle, and provides an angle difference signal. The correcting section determines whether the vehicle is in an under steer state or in an over steer state, based on a direction of the angle difference signal and a direction of the yaw rate signal, and sets an appropriate correction amount corresponding to each state. The correcting section also determines the under steer state, the over steer state, or the counter steer state, based on a direction of the angle difference signal, a direction of the differential value of the angle difference signal, a direction of the yaw rate signal, and a direction of the steering wheel torque from the steering wheel torque sensor, and sets an appropriate correction amount corresponding to each state. The correcting section further corrects the correction amount by increasing or decreasing to the target torque signal. The electric power steering apparatus therefore informs the driver of a change of the road reaction force via the steering wheel, by way of changing an assist torque in response to the under steer state, the over steer state, or the counter steer state.
However, in the electric power steering apparatus disclosed in Japanese Patent Laid-open Publication No. HEI-11-152057, while it can judge the under steer state or the over steer state from the angle difference signal, irrespective of a road surface friction coefficient, there may arise a discrepancy between the angle difference signal and the actual vehicle behavior. In other words, when detecting the vehicle state based on the angle difference signal, there is a case in which the vehicle state is not accurately detected.
Generally, in a region where the steer angle (turn angle of the front wheels) is greater, the vehicle is set for under steer (weak under steer). Therefore, when the vehicle is in a condition that the angle difference between the front wheel slip angle and the rear wheel slip angle is 0 (deg), i.e. in the neutral steer state, the vehicle is actually in a shifting or proceeding state just before shifting to the over steer state. As to the vehicle behavior, the under steer state is more stable than the over steer state. The vehicle is thus controlled so as not to be in the over steer state. For this reason, the target torque signal is preferably corrected in the shifting state, for the preparation of the over steer state. Especially in a sport car with smaller yaw inertial mass, the correction of the target torque signal should be made during the shifting state, otherwise the counter operation may not be made in time. In the aforementioned electric power steering apparatus, however, because the steer angle (turn angle of the front wheels) is not employed as a parameter for judging the vehicle states, accurate judging for this shifting state cannot be made. Therefore, this electric power steering apparatus corrects the target torque signal after shifting to the over steer state. Meanwhile, in a region where the steer angle is greater, the yaw rate becomes greater, leading to susceptibility to the over steer condition. For this reason, the target torque signal is preferably corrected swiftly during the shifting from the under steer state to the neutral steer state (conventionally, it is judged as the under steer condition) and as the steer angle is getting greater (i.e., in a region where the absolute value between the front wheel slip angle and the rear wheel slip angle is large).
Further, when the vehicle speed is faster or when the road surface friction coefficient is low, the vehicle tends to shift from the under steer state to a drift out state, due to a decreased lateral force. Especially for a heavy-weighted vehicle with greater yaw inertial mass, the vehicle, is liable to shift to the drift out state. Shifting to the drift out state (excessive under steer state) is determined not only by the angle difference between the front wheel slip angle and the rear wheel slip angle but also by taking the steer angle into consideration. Specifically, the smaller the steer angle, the more likely to shift to the drift out state in a region where the angle difference between the front wheel slip angle and the rear wheel slip angle is smaller. However, because the conventional electric power steering apparatus does not employ steer angle (turn angle of the front wheels) as a parameter to judge the vehicle state, an accurate judgement cannot be made whether or not shifting to the drift out state.
Meanwhile, the four wheel steering apparatus effectively turns the rear wheels, when the vehicle runs straight in which the target yaw rate is small, so as to restrict the actual yaw rate. However, when the target yaw rate is large, the four wheel steering apparatus has to set the turning angle of the rear wheels in accordance with the road surface friction coefficient. For this reason, there is a need for highly accurately detecting the road surface friction coefficient, which involves difficulty in detection of the friction coefficient directly and complicated calculations to predict the friction coefficient. Further, left-and-right driving force distributing apparatus cannot most effectively control the braking force when the yaw rate is greater.
With the foregoing drawbacks of the prior art in view, the present invention seeks to provide a vehicle behavior control apparatus, which accurately detects the vehicle state and controls the vehicle behavior in accordance with the detected vehicle state.
According to the present invention, there is provided a vehicle behavior control apparatus comprising:
a slip angle difference predicting section for predicting a difference between a slip angle of front wheels and a slip angle of rear wheels;
a steer angle detecting section for detecting a steer angle of the vehicle; and
a control section for controlling turning behaviors of the vehicle, based on an angle difference signal from said slip angle difference predicting section and a steer angle signal from said steer angle detecting section.
With this vehicle behavior control apparatus, the input-output relations of the vehicle are detected by the steer angle signal that is an input signal from the driver to the vehicle and by the angle difference signal that is an output signal from the vehicle, leading to accurate detection of the vehicle state. Therefore, the vehicle behavior control apparatus appropriately controls turning behaviors of the vehicle in accordance with the detected vehicle state.
There is also provided a vehicle behavior control apparatus comprising:
a steering torque sensor for detecting a steering torque of a steering system;
an electric motor for applying an assist torque to the steering system;
a control unit having a target torque signal setting section for setting a target torque signal based on a steering torque signal from said steering torque sensor; and
a motor driver for driving said electric motor based on said target torque signal,
wherein the vehicle behavior control apparatus further comprises a slip angle difference predicting section for predicting a difference between a slip angle of front wheels and a slip angle of rear wheels, and a steer angle detecting section for detecting a steer angle of the vehicle, and said control unit has a correcting section for correcting said target torque signal based on an angle difference signal from said slip angle difference predicting section and a steer angle signal from said steer angle detecting section, so as to control turning behavior of the vehicle.
With this vehicle behavior control apparatus, the input-output relations of the vehicle are detected by the steer angle signal that is an input signal from the driver to the vehicle and by the angle difference signal that is an output signal from the vehicle, leading to accurate detection of the vehicle state. Further, the vehicle behavior control apparatus sets a correction amount at the correcting section, in accordance with the vehicle state to be detected, and generates an assist torque based on the target torque signal taking into consideration the correction amount. As a result, by this corrected assist torque, a change of the road reaction force is transmitted accurately to the driver via a steering wheel, and the vehicle behavior is stabilized by a steering operation in accordance with an intention of the driver.
Further, in the above vehicle behavior control apparatus, said correcting section sets a correction amount to correct said target torque signal, based on an angle difference signal from said slip angle difference predicting section and a yaw rate signal to be detected at a yaw rate detecting section.
With this vehicle behavior control apparatus, because the correcting section sets the correction amount, based on the angle difference signal and the yaw rate signal, the correction amount can be set such that both the angle difference signal and the yaw rate signal are decreased to zero. The vehicle behavior control apparatus then urges a steering operation of the driver, by the assist torque taking into consideration the correction amount, so as to decrease the steer angle. As a result, because both the angle difference signal and the yaw rate signal decrease to zero, the vehicle behavior is more stabilized.
Further, in the above vehicle behavior control apparatus, said slip angle difference predicting section calculates said angle difference signal based on a steer angle signal from said steer angle detecting section, a vehicle speed signal to be detected at a vehicle speed sensor, a yaw rate signal to be detected at the yaw rate detecting section, and dimensional parameters of the vehicle.
With this vehicle behavior control apparatus, the angle difference can be predicted by using the existing sensors mounted on the vehicle without detecting the actual angle difference. Furthermore, because parameters for calculating the angle difference are detected directly, the accuracy of the predicted angle difference becomes higher.